Piston ring

ABSTRACT

According to a first embodiment of the present invention, a piston ring  1  is provided that has a stainless steel base material  2  having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, the surfaces each having a nitride layer  3  formed thereon, where the nitride layer  3  each is first formed to have a compound layer  4  and a diffusion layer  5,  where the diffusion layer  5  of the outer circumferential surface is then exposed by removing the compound layer  4  of the outer circumferential surface, where the exposed diffusion layer  5  has thereafter an ion-plating deposition layer  8  of hard ceramic formed thereon. The piston ring  1  is characterized in that the exposed diffusion layer  5  is removed on the upper and lower corners  7  of the upper and lower surfaces and the outer circumferential surface by chamfering corner regions  6  located around the upper and lower corners  7  before the ion-plating deposition layer  8  is formed. According to a second embodiment of the present invention, a piston ring  1   a  is provided that has a stainless steel base material  2   a  having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, the surfaces each having a nitride layer  3   a  formed thereon, where the nitride layer  3   a  each is first formed to have a compound layer  4   a  and a diffusion layer  5   a,  where the diffusion layer  6   a  of the outer circumferential surface is then exposed by removing the compound layer  4   a  of the outer circumferential surface, where the exposed diffusion layer  5   a  has thereafter an ion plating deposition layer  8   a  of hard ceramic formed thereon. The piston ring  1   a  is characterized in that the exposed diffusion layer  45   a  is reduced in the thickness thereof to 30% or less of the nitride layer in the thickness thereof on the upper and lower corners  7   a  of the upper and lower surfaces and the outer circumferential surface by chamfering corner regions  6   a  located around the upper and lower corners  7   a  before the ion-plating deposition layer  8   a  is formed.  
     With a piston ring  1, 1   a  implemented as described above, advantages are effected in that degradation of the fatigue strength is prevented that otherwise would be caused by cracks occurring in the ion-plating deposition layer  8, 8   a  of such piston ring  1, 1   a;  better adhesion is obtained compared with the ion-plating deposition layer formed directly on the stainless steel base material, as in prior art; scuffing would not occur even if the ion-plating deposition layer on the outer circumferential surface wears out.

FIELD OF THE INVENTION

[0001] The present invention relates to a stainless steel piston ring having excellent abrasion resistance.

BACKGROUND OF THE INVENTION

[0002] Conventional stainless steel piston rings of excellent abrasion resistance are of the type in which the piston rings have a nitride layer formed on the upper and lower surfaces and the inner and outer circumferential surfaces of the stainless steel base material of the piston rings, by applying the nitriding process to those surfaces, where the diffusion layer of the nitride layer on the outer circumferential surface is then exposed by removing the compound layer of the nitride layer on the outer circumferential surface, and, thereafter, the exposed diffusion layer has an ion-plating deposition layer of hard ceramic formed thereon by vacuum vapor depositing hard ceramic thereon.

[0003] This type of piston ring had a disadvantage in that, when cracks occur in the ion-plating deposition layer, the cracks would propagate to the nitride layer, thereby resulting in degradation of the fatigue strength.

[0004] In order to overcome this disadvantage, a piston ring is proposed in the Japanese Patent Publication No. 3,090,520 in which the piston ring has only an ion-plating deposition layer formed directly on the outer circumferential surface of the stainless steel base material thereof, where the nitride layer is only formed on the upper and lower surfaces and the inner circumferential surface of the stainless steel base material.

[0005] The ion-plating deposition layer formed directly on the stainless steel base material however has a problem in that it is inferior in adhesion to an ion-plating deposition layer formed on the diffusion layer, of a nitride layer on the stainless steel base material, exposed by removing the compound layer of the nitride layer. Furthermore, another problem exists in that, in case an ion-plating deposition layer is formed directly on the outer circumferential surface of the stainless steel base material, the stainless steel base material tends to slide directly in contact with the surface of a cylinder or cylinder liner, that is otherwise supposed to slide in contact with the surface of the ion-plating deposition layer, as the ion-plating deposition layer wears out, thereby causing scuffing or scuff-marks, where countermeasures against it of increasing the deposition layer thickness would cause degradation of the breakage resistance, and, at the same time, lead to an increase in the manufacturing costs.

[0006] The present invention has been made to solve these problems. Therefore, an object of the present invention is to provide a piston ring of the type in which a nitride layer is formed on the upper and lower surfaces and the inner and outer circumferential surfaces of the stainless steel base material of a piston ring, where an ion-plating deposition layer of hard ceramic is formed on the nitride layer of the outer circumferential surface, wherein the piston ring has an advantage in that degradation of the fatigue strength would not be caused even when cracks occur in the ion-plating deposition layer.

[0007] The present inventors have come to think of, and devise, the principle of the present invention by recognizing that, although cracks cannot be prevented from occurring in the ion-plating deposition layer on the outer circumferential surface of a piston ring because the ion-plating deposition layer is lacking in toughness, degradation of the fatigue strength of the piston ring caused by such cracks could be prevented by restraining the propagation of cracks occurring in the ion-plating deposition layer on the upper and lower corner regions of the upper and lower surfaces and the outer circumferential surface, because the cracks mainly occur on these corners.

SUMMARY OF THE INVENTION

[0008] In order to accomplish the object described above, a piston ring is provided, according to a first embodiment of the present invention, that is an improved piston ring of the type that has a stainless steel base material having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, the surfaces each having a nitride layer formed thereon, where the nitride layer each is first formed to have a compound layer and a diffusion layer, where the diffusion layer of the outer circumferential surface is then exposed by removing the compound layer of the outer circumferential surface, where the exposed diffusion layer has thereafter an ion-plating deposition layer of hard ceramic formed thereon by vacuum vapor depositing hard ceramic thereon. The piston ring, according to the first embodiment of the present invention, of the type described above is then characterized in that, as the improvement, the exposed diffusion layer is removed on the upper and lower corners of the upper and lower surfaces and the outer circumferential surface by chamfering corner regions located around the upper and lower corners before the ion-plating deposition layer is formed.

[0009] According to a second embodiment of the present invention, a piston ring is provided that is another improved piston ring of the type described above with the first embodiment of the present invention. The piston ring, according to the second embodiment of the present invention, of the type described above is then characterized in that, as the improvement, the exposed diffusion layer is reduced in the thickness thereof to 30% or less of the nitride layer in the thickness thereof on the upper and lower corners of the upper and lower surfaces and the outer circumferential surface by chamfering corner regions located around the upper and lower corners before the ion-plating deposition layer is formed.

[0010] When a piston ring is arranged as described above, according to the present invention, cracks tends to occur in the ion-plating deposition layer on the upper and lower corner regions of the stainless steel base material of the piston ring, as in conventional piston rings, because the ion-plating deposition layer is lacking in toughness. In the piston ring according to the present invention, however, even when cracks actually occur in the ion-plating deposition layer, such cracks would not propagate to areas below the deposition layer, because, right under the ion-plating deposition layer, there exists a zone that would not allow cracks to propagate therethrough, that is, depending on cases, either the non-nitrided region of the stainless steel base material or an area located deepest in the diffusion layer that is little nitrided.

[0011] Specifically, in case that, according to the first embodiment of the present invention, the upper and lower corner regions of the nitride layer of the piston ring is chamfered deep enough to remove an entire nitride layer to expose the stainless steel base material on the upper and lower corner regions thereof, cracks occurring in the ion-plating deposition layer are definitely unlikely to propagate below because the non-nitrided stainless steel base material that would not allow cracks to propagate therethrough is located right under the ion-plating deposition layer.

[0012] However, in case that the axial length or thickness of the piston ring is relatively small, whereby deep chamfering could deteriorate the sealing capability of the piston ring, then, preferably, the corner chamfering process is conducted to the extent such that the exposed diffusion layer remains in the thickness thereof equal to 30% or less of the nitride layer in the original thickness thereof on the upper and lower corner regions of the outer circumferential surface, according to the second embodiment of the present invention.

[0013] In order to improve the crack preventive capability of the ion-plating deposition layer, the crystal state of the layer is preferably arranged so as to have a columnar structure with the crystal orientation at [200].

[0014] With a piston ring implemented as described above, an advantage is effected in that degradation of the fatigue strength is prevented that otherwise would be caused by cracks occurring in the ion-plating deposition layer of such piston ring.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is cross-sectional views of a piston ring according to a first embodiment of the present invention, where view (a) shows the piston ring before chamfering, view (b) shows the piston ring after chamfering, and view (c) shows the piston ring after an ion-plating deposition layer is formed.

[0016]FIG. 2 is cross-sectional views of a piston ring, similar to FIG. 1 above but according to a second embodiment of the present invention, where view (a) shows the piston ring after chamfering, and view (b) shows the piston ring after an ion-plating deposition layer is formed.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring to FIG. 1, an illustrative piston ring, according to the present invention, are described hereunder. FIG. 1 is cross-sectional views of a piston ring 1 according to a first embodiment of the present invention, where view (a) shows the piston ring 1 before chamfering, view (b) shows the piston ring 1 after chamfering, and view (c) shows the piston ring 1 after an ion-plating deposition layer 8 is formed.

[0018] As shown in FIG. 1(a), the piston ring 1 has a stainless steel base material 2 having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, where all the surfaces has a nitride layer 3 formed thereon, where the nitride layer 3 has a thickness of 60 to 120 μm. The region of surface and right under the surface of the nitride layer 3 is a compound layer 4 having a high nitrogen concentration, under which exists a diffusion layer 5, where the nitrogen concentration is gradually reduced along the depth of the layer.

[0019] As shown in FIG. 1(b), the exposed diffusion layer 5 of the piston ring 1 is entirely removed on the upper and lower corners 7 of the upper and lower surfaces and the outer circumferential surface by chamfering corner regions 6 located around the upper and lower corners of the outer circumferential surface by means of grinding, thereby entirely removing the nitride layer 3 and exposing the stainless steel base material 2 on the upper and lower corners 7. Before or after such chamfering, the compound layer 4 of the outer circumferential surface is also removed from the nitride layer 3 of the upper and lower surfaces and the outer circumferential surface of the piston ring 1, also by means of grinding, thereby exposing the diffusion layer 5 of the nitride layer 3.

[0020] Then, as shown in FIG. 1(c), an ion-plating deposition layer 8 of hard ceramic is formed on the outer circumferential surface of the diffusion layer 5 of the piston ring 1 by means of vacuum vapor depositing. The thickness of the diffusion layer 5 located under the ion-plating deposition layer 8 is approximately constant except the upper and lower corner regions 6 of the outer circumferential surface, where the thickness decreases as nearing the upper and lower corners 7, and is equal to zero at the upper and lower corners 7.

[0021] When this piston ring 1 arranged as above is used, cracks may occur in the ion-plating deposition layer 8 on the upper and lower corner regions 6 of the stainless steel base material 2 of the piston ring 1 because the ion-plating deposition layer 8 is lacking in toughness, and such cracks may act on the stainless steel base material 2 or the diffusion layer 5 near the upper and lower corners 7 thereof. However, because the stainless steel base material 2 is not affected by diffused nitrogen except the nitride layer 3, and because the region of the diffusion layer 5 on and near the upper and lower corners 7 is located deepest in the nitrogen layer 3 where the diffused nitrogen concentration is relatively low, thereby effect of the diffused nitrogen being relatively little, cracks thus would not propagate to the stainless steel base material 2 or the diffusion layer 5. Therefore, the cracks occurring in the ion-plating deposition layer 8 is unlikely to result in degradation of the fatigue strength of the piston ring 1.

[0022] In the piston ring 1, according to the present invention, the ion-plating deposition layer 8 is formed on the diffusion layer 5, thereby having better adhesion compared with the ion-plating deposition layer formed directly on the stainless steel base material. Thus, the ion-plating deposition layer 8, according to the present invention, is unlikely to peel off. Furthermore, even if the ion-plating deposition layer 8 on the outer circumferential surface wears out, scuffing would not occur because the diffusion layer 5 of the nitride layer 3 is located right under the ion-plating deposition layer 8.

[0023] While the compound layer 3 of the upper and lower surfaces has been arranged to be removed after the nitriding process with the illustrative piston ring 1 described according to FIG. 1, such compound layer 3 of the upper and lower surfaces may be left unremoved.

[0024] Next, referring to FIG. 2, another illustrative piston ring, according to the present invention, are described hereunder.

[0025]FIG. 2 is cross-sectional views of a piston ring 1 a, similar to FIG. 1 but according to a second embodiment of the present invention, having the axial length relatively small compared with the radial length thereof, where view (a) shows the piston ring 1 a after chamfering, and view (b) shows the piston ring 1 a after an ion-plating deposition layer 8 a is formed.

[0026] First, the piston ring 1 a has a nitride layer 3 a formed on all the surfaces of a stainless steel base material 2 a, similar to the piston ring shown in FIG. 1(a), where the nitride layer 3 a has a thickness of 60 to 120 Am. As shown in FIG.2(a), the compound layer 4 a of the outer circumferential surface is removed from the nitride layer 3 a of the outer circumferential surface of the piston ring 1 a, to expose the diffusion layer 5 a on the outer circumferential surface, similar to FIG. 1(b). The exposed diffusion layer 5 a is reduced in the minimum thickness thereof to 30% or less of the nitride layer 3 a in the thickness thereof on the upper and lower corners of the outer circumferential surface, for example, the diffusion layer 5 a being 20 μm when the nitride layer 3 a 100 μm, by chamfering corner regions 6 a located around the upper and lower corners 7 a.

[0027] The, as shown in FIG. 2(b), an ion-plating deposition layer 8 a of hard ceramic is formed on the outer circumferential surface of the diffusion layer 5 a of the piston ring 1 a. Also, as shown in FIG. 2(b), The thickness of the diffusion layer 5 a located under the ion-plating deposition layer 8 a is approximately constant except the upper and lower corner regions 6 a of the outer circumferential surface, where the thickness gradually decreases to 30% of less of the thickness of the nitride layer 3 a as nearing the upper and lower corners 7 a.

[0028] When this piston ring 1 a arranged as above is used, cracks may occur in the ion-plating deposition layer 8 a on the upper and lower corner regions 6 a near the upper and lower corners 7 a of the stainless steel base material 2 a of the piston ring 1 a because the ion-plating deposition layer 8 a is lacking in toughness, and such cracks may act on the diffusion layer 5 a near the upper and lower corners 7 a thereof. However, because the region of the diffusion layer 5 a on and near the upper and lower corners 7 a is located deepest in the nitrogen layer 3 a where the diffused nitrogen concentration is relatively low, cracks thus would not propagate further. Therefore, the cracks occurring in the ion-plating deposition layer a 8 a is unlikely to result in degradation of the fatigue strength of the piston ring 1 a. While the compound layer 3 a of the upper and lower surfaces has been arranged to remain after the nitriding process with the illustrative piston ring 1 a described according to FIG.2, such compound layer 3 a of the upper and lower surfaces may be removed, as described according to FIG. 1.

[0029] The arrangements and effect of this illustrative piston ring 1 a, other than the description above is similar to the illustrative piston ring 1 according to FIG. 1.

[0030] As described above, a piston ring arranged according to the present invention, has an excellent advantage in that degradation of the fatigue strength of the piston ring is unlikely to result from cracks, first occurring in the ion-plating deposition layer, and otherwise further propagating to areas below the deposition layer, because, right under the ion-plating deposition layer, a zone is arranged to exist that would not allow cracks to propagate therethrough, where, depending on cases, the zone is either the non-nitrided region of the stainless steel base material or an area located deepest in the diffusion layer that is little nitrided.

[0031] The piston ring arranged according to the present invention, has another excellent advantage in that better adhesion is obtained compared with the ion-plating deposition layer formed directly on the stainless steel base material, as in prior art, because the ion-plating deposition layer is formed on the diffusion layer, whereby the ion-plating deposition layer, according to the present invention, is unlikely to peel off.

[0032] Furthermore, the piston ring arranged according to the present invention, has another excellent advantage in that scuffing would not occur even if the ion-plating deposition layer on the outer circumferential surface wears out, because the diffusion layer of the nitride layer is located right under the ion-plating deposition layer, thereby preventing the stainless steel base material of the piston ring from directly contacting the inner wall of the cylinder or cylinder liner that the piston ring is adapted to slide on.

[0033] While there have been shown and described fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various modifications and changes to such embodiments may be made by those skilled in the art without departing from the spirit of the invention. It is our intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 

What is claimed is:
 1. A piston ring (1) having a stainless steel base material (2), said base material (2) having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, said surfaces each having a nitride layer (3) formed thereon, said nitride layer (3) each first being formed to have a compound layer (4) and a diffusion layer (5), said diffusion layer (5) of said outer circumferential surface then being exposed by removing said compound layer (4) of said outer circumferential surface, said exposed diffusion layer (5) thereafter having an ion-plating deposition layer (8) of hard ceramic formed thereon, said piston ring (1) CHARACTERIZED IN THAT: said exposed diffusion layer (5) is entirely removed on the upper and lower corners (7) of said upper and lower surfaces and said outer circumferential surface by chamfering corner regions (6) of said nitride layer (3) located around said upper and lower corners (7) before said ion-plating deposition layer (8) is formed.
 2. A piston ring (1 a) having a stainless steel base material (2 a), said base material (2 a) having an upper surface, a lower surface, an inner circumferential surface and an outer circumferential surface, said surfaces each having a nitride layer (3 a) formed thereon, said nitride layer (3 a) each first being formed to have a compound layer (4 a) and a diffusion layer (5 a), said diffusion layer (5 a) of said outer circumferential surface then being exposed by removing said compound layer (4 a) of said outer circumferential surface, said exposed diffusion layer (5 a) thereafter having an ion-plating deposition layer (8 a) of hard ceramic formed thereon, said piston ring (1 a) CHARACTERIZED IN THAT: said exposed diffusion layer (5 a) is reduced in the thickness thereof to 30% or less of said nitride layer (3 a) in the thickness thereof on the upper and lower corners (7 a) of said upper and lower surfaces and said outer circumferential surface by chamfering corner regions (6 a) of said nitride layer (3 a) located around said upper and lower corners (7 a) before said ion-plating deposition layer (8 a) is formed. 